1307.1702
Growth of brightest cluster galaxies via mergers since z=1
Burke, Collins
Hierarchical assembly within clusters of galaxies is tied directly to the evolution of the BCGs, which dominate the stellar light in the centres of rich clusters. Investigate the number of mergers onto BCGs in 14 X-ray selected clusters over 0.8<z<1.4 using HST imaging data. Find significant differences in the numbers of companion galaxies to BCGs between the clusters in our sample, indicating that BCGs in similar mass clusters can have very different merging histories. Within a 50 kpc radius around the BCGs, find an average of 6.45 pm 1.15 companion galaxies with mass ratios between 1:1 and 1:20. The infalling companions show a 50/50 split between major (1:1-1:2) and minor (1:3-1:20) mergers. When compared to similar work using lower redshift clusters, these results demonstrate that both major and minor merging was more common in the past [so what does the local cluster infall companions look like?]. Since the dynamical timescales for merging onto the BCG are relatively short compared with the look-back time to z~1, results suggest that the BCG stellar mass may increase by as much as 1.8 times since z=1. However, the growth rate of BCGs will be substantially less if stripped material from nearby companions ends up in the diffuse ICL.
1307.2265
Enzo: An adaptive mesh refinement code for astrophysics
The Enzo collaboration
Description of the AMR code Enzo.
1307.2355
A comparison of CMB lensing efficiency of gravitational waves and large scale structure
Padmanabhan, Rotti, Souradeep
Comparison of the WL effects due to LSS, or scalar density perturbations and those due to GW or tensor perturbations, on the temperature and polarization power spectra of the CMB. Analysis both in real space (correlation fn) and spherical harmonic space. Similarity between the lensing kernels associated with LSS lensing and GW lensing. Found that the lensing kernels only differ in relative negative signs and their form is reminiscent of even and odd parity bipolar spherical harmonic coefficients. Numerical study of these kernels show that lensing due to GW is more efficient at distorting the CMB spectra as compared to LSS lensing, particularly for the polarization power spectra. Argue that the CMB B-mode PS measurements can be used to place interesting constraints on GW energy densities.
1307.2377
How real-time cosmology can distinguish between different anisotropic models
Amendola et al
New analysis on how to distinguish between isotropic and anisotropic cosmological models based on tracking the angular displacements of a large number of distant quasars over an extended period of time, and then performing a multiple-vector decomposition of the resulting displacement maps. Find that while the GAIA mission operating at its nominal specifications does not have sufficient angular resolution to resolve anisotropic universes from isotropic ones within 10 years, the next-generation GAIA-like survey with a resolution 10x better should be equal to the task. Distinguishing between different anisotropic models is however more demanding. Keeping the observational timespan to 10 years, find that the angular resolution of the survey will need to be of order 0.1 micro-arcsecond in order for certain rotating anisotropic models to produce a detectable signature. Should such a detection become possible, it would immediately allow ruling out large local void models.
1307.2903
PST-CLJ2040-4451: an SZ-selected galaxy cluster at z=1.478 with significant ongoing star formation
Bayliss et al
Highest z galaxy cluster discovered via SZ, spectroscopically confirmed. Found in SPT 720 deg^2 survey. 15 member galaxy's spectra, all have strong [O II] 3727 emission. Mass of M_500,SZ=3.2e14 Msun/h_70, corresponding to M_200,SZ=5.8e14 Msun/h_70. The velocity dispersion measured entirely from blue star forming members is sigma_v=1500pm520 km/s. This massive, high-z cluster is experiencing a phase of active SF, and suports recent results showing a marked increase in SF occurring in galaxy clusters at z>1.4. Also compute the probability of finding a cluster as rare as this as >99.5% (i.e., pretty common cluster for a LCDM model).
1307.2906
Redshift space distortion reconstruction
Vallinotto, Linder
z-space distortions of matter PS carry info on the growth rate of cosmic structure, but require accurate modeling of NL and velocity effects on the density field. Test and advance the reconstruction function of Kwan, Lewis, Linder in a few ways: 1. compare to the distribution function perturbative approach of Seljak+McDonald, deriving the mapping between them and showing how the KLL form can extend the validity of the latter. 2. Using cosmological simulations rather than perturbation theory, calibrate the free functions within the KLL reconstruction form and fit their behavior in redshift and wavenumber. An efficient, new analysis pipeline calculates from simulation outputs the redshift space PS. The end result is a robust analytic reconstruction mapping the linear, real space matter PS to the NL, anisotropic redshift space power spectrum to an accuracy of ~2-5% in the range k=0.1-0.5 h?Mpc for z=0-2. Derive RealEasy, an analytic mapping to the NL, real space matter PS (an analog to Halofit) to ~1% accuracy over the same range.
1307.2896
Equilibrium star formation in a constant Q disk: model optimisation and initial tests
Zheng, et al
Develop a model for the distribution of the ISM and SF in galaxies based on recent studies that indicate that galactic disks stabilize to a constant stability parameter; combine this with prescriptions of how the phases of the ISM are determined and for the star formation law (SFL). The model predicts the gas surface mass density and SF intensity of a galaxy given its rotation curve, stellar surface mass density and the gas velocity dispersion. This model is tested on radial profiles of neutral and molecular ISM surface mass density and SF intensity of 12 galaxies selected from the THINGS sample. Tests focus on intermediate eradii. The model produces reasonable agreement with ISM mass and SFR integrated over the central region in all but one case. To optimise the model, evaluate 4 recipes for the stability parameter, but 3 recipes for apportioning the ISM into molecular and neutral components, and 8 versions of the SFL. Find no clear-cut best prescription for the 2fluid (gas and stars) stability parameter Q_2f and therefore for simplicity, use the Wang+Silk approximation (Q_WS). Find that an empirical scaling between the molecular to neutral ISM ratio (R_ml) and the stellar surface mass density proposed by Leroy+ works marginally better than the tother two prescriptions for this ratio in predicting in the ISM profiles, and noticeably better in predicting SF intensity from the ISM profiles produced by model with the SFLs tested. Thus in the context of the modeled ISM profiles, the linear molecular SFL and the 2-component SFL (Krumholz+) work better than the other prescriptions tested. Incorporate these relations into the CQ-disk model.
1307.2900
Spoon-feeding giant stars to supermassive black holes: episodic Roche lobe overflow from evolving stars and their contribution to the quiescent activity of galactic nuclei
MacLeod ... Guillochon, et al
Explore a gradual process of tidal stripping where stars approach the tidal disruption radius by stellar evolution while in an eccentric orbit. After the onset of mass transfer, these stars episodically overflow their Roche lobes every pericenter passage giving rise to low-level flares that repeat on the orbital timescale. Giant stars will exhibit a runaway response to mass loss and "spoon-feed" material to the BH for tens to hundreds of orbital periods. In contrast to full tidal disruption events, the duty cycle of this feeding mode is of order unity for BHs with mass greater than approximately 10 million solar masses. This mode of quasi-steady SMBH feeding is competitive with indirect SMBH feeding through stellar winds, and spoon-fed giant stars may play a significant role in determining the quiescent luminosity of local SMBHs.
1307.3049
A comparative study of radio halo occurrence in SZ and X-ray selected galaxy cluster samples
Sommer, Basu
Aim to make an unbiased census of the radio halo population in galaxy clusters and test whether current low number counts of radio haloes have been a result of selection biases. Construct near-complete samples of galaxy clusters based on public X-ray and SZ cluster catalogs, and search for diffuse, extended (Mpc-scale) emission near their center by analyzing data from the NRAO very large array sky survey. Remove point sources using a matched filtering algorithm, and model the diffuse emission by radial fitting and direct integration. Use and extensive suite of simulations to check for biases. Model the relation between mass and radio halo power with a power law, accounting for the possibility of a population hosting no radio halo emission. Constrain the fraction of targets hosting an extended emission at their centers at 1.4 GHz. Find power law slopes consistent with previous works, though findings suggest the fraction of targets hosting radio haloes may have to be revised upward when considering clusters selected in the SZ effect: ~65% of X-ray selected targets are found to contain no extended radio emission, the corresponding fraction in the SZ selected samples is roughly 20%. The "radio-quiet" population in the X-ray selected cluster samples is in agreement with previous findings. Propose a simple explanation for the observed selection difference based on the distinct time evolution of the SZ and X-ray observables during cluster mergers, and a bias towards relaxed, cool-core clusters in the X-ray selected samples.
Growth of brightest cluster galaxies via mergers since z=1
Burke, Collins
Hierarchical assembly within clusters of galaxies is tied directly to the evolution of the BCGs, which dominate the stellar light in the centres of rich clusters. Investigate the number of mergers onto BCGs in 14 X-ray selected clusters over 0.8<z<1.4 using HST imaging data. Find significant differences in the numbers of companion galaxies to BCGs between the clusters in our sample, indicating that BCGs in similar mass clusters can have very different merging histories. Within a 50 kpc radius around the BCGs, find an average of 6.45 pm 1.15 companion galaxies with mass ratios between 1:1 and 1:20. The infalling companions show a 50/50 split between major (1:1-1:2) and minor (1:3-1:20) mergers. When compared to similar work using lower redshift clusters, these results demonstrate that both major and minor merging was more common in the past [so what does the local cluster infall companions look like?]. Since the dynamical timescales for merging onto the BCG are relatively short compared with the look-back time to z~1, results suggest that the BCG stellar mass may increase by as much as 1.8 times since z=1. However, the growth rate of BCGs will be substantially less if stripped material from nearby companions ends up in the diffuse ICL.
1307.2265
Enzo: An adaptive mesh refinement code for astrophysics
The Enzo collaboration
Description of the AMR code Enzo.
1307.2355
A comparison of CMB lensing efficiency of gravitational waves and large scale structure
Padmanabhan, Rotti, Souradeep
Comparison of the WL effects due to LSS, or scalar density perturbations and those due to GW or tensor perturbations, on the temperature and polarization power spectra of the CMB. Analysis both in real space (correlation fn) and spherical harmonic space. Similarity between the lensing kernels associated with LSS lensing and GW lensing. Found that the lensing kernels only differ in relative negative signs and their form is reminiscent of even and odd parity bipolar spherical harmonic coefficients. Numerical study of these kernels show that lensing due to GW is more efficient at distorting the CMB spectra as compared to LSS lensing, particularly for the polarization power spectra. Argue that the CMB B-mode PS measurements can be used to place interesting constraints on GW energy densities.
1307.2377
How real-time cosmology can distinguish between different anisotropic models
Amendola et al
New analysis on how to distinguish between isotropic and anisotropic cosmological models based on tracking the angular displacements of a large number of distant quasars over an extended period of time, and then performing a multiple-vector decomposition of the resulting displacement maps. Find that while the GAIA mission operating at its nominal specifications does not have sufficient angular resolution to resolve anisotropic universes from isotropic ones within 10 years, the next-generation GAIA-like survey with a resolution 10x better should be equal to the task. Distinguishing between different anisotropic models is however more demanding. Keeping the observational timespan to 10 years, find that the angular resolution of the survey will need to be of order 0.1 micro-arcsecond in order for certain rotating anisotropic models to produce a detectable signature. Should such a detection become possible, it would immediately allow ruling out large local void models.
1307.2903
PST-CLJ2040-4451: an SZ-selected galaxy cluster at z=1.478 with significant ongoing star formation
Bayliss et al
Highest z galaxy cluster discovered via SZ, spectroscopically confirmed. Found in SPT 720 deg^2 survey. 15 member galaxy's spectra, all have strong [O II] 3727 emission. Mass of M_500,SZ=3.2e14 Msun/h_70, corresponding to M_200,SZ=5.8e14 Msun/h_70. The velocity dispersion measured entirely from blue star forming members is sigma_v=1500pm520 km/s. This massive, high-z cluster is experiencing a phase of active SF, and suports recent results showing a marked increase in SF occurring in galaxy clusters at z>1.4. Also compute the probability of finding a cluster as rare as this as >99.5% (i.e., pretty common cluster for a LCDM model).
1307.2906
Redshift space distortion reconstruction
Vallinotto, Linder
z-space distortions of matter PS carry info on the growth rate of cosmic structure, but require accurate modeling of NL and velocity effects on the density field. Test and advance the reconstruction function of Kwan, Lewis, Linder in a few ways: 1. compare to the distribution function perturbative approach of Seljak+McDonald, deriving the mapping between them and showing how the KLL form can extend the validity of the latter. 2. Using cosmological simulations rather than perturbation theory, calibrate the free functions within the KLL reconstruction form and fit their behavior in redshift and wavenumber. An efficient, new analysis pipeline calculates from simulation outputs the redshift space PS. The end result is a robust analytic reconstruction mapping the linear, real space matter PS to the NL, anisotropic redshift space power spectrum to an accuracy of ~2-5% in the range k=0.1-0.5 h?Mpc for z=0-2. Derive RealEasy, an analytic mapping to the NL, real space matter PS (an analog to Halofit) to ~1% accuracy over the same range.
1307.2896
Equilibrium star formation in a constant Q disk: model optimisation and initial tests
Zheng, et al
Develop a model for the distribution of the ISM and SF in galaxies based on recent studies that indicate that galactic disks stabilize to a constant stability parameter; combine this with prescriptions of how the phases of the ISM are determined and for the star formation law (SFL). The model predicts the gas surface mass density and SF intensity of a galaxy given its rotation curve, stellar surface mass density and the gas velocity dispersion. This model is tested on radial profiles of neutral and molecular ISM surface mass density and SF intensity of 12 galaxies selected from the THINGS sample. Tests focus on intermediate eradii. The model produces reasonable agreement with ISM mass and SFR integrated over the central region in all but one case. To optimise the model, evaluate 4 recipes for the stability parameter, but 3 recipes for apportioning the ISM into molecular and neutral components, and 8 versions of the SFL. Find no clear-cut best prescription for the 2fluid (gas and stars) stability parameter Q_2f and therefore for simplicity, use the Wang+Silk approximation (Q_WS). Find that an empirical scaling between the molecular to neutral ISM ratio (R_ml) and the stellar surface mass density proposed by Leroy+ works marginally better than the tother two prescriptions for this ratio in predicting in the ISM profiles, and noticeably better in predicting SF intensity from the ISM profiles produced by model with the SFLs tested. Thus in the context of the modeled ISM profiles, the linear molecular SFL and the 2-component SFL (Krumholz+) work better than the other prescriptions tested. Incorporate these relations into the CQ-disk model.
1307.2900
Spoon-feeding giant stars to supermassive black holes: episodic Roche lobe overflow from evolving stars and their contribution to the quiescent activity of galactic nuclei
MacLeod ... Guillochon, et al
Explore a gradual process of tidal stripping where stars approach the tidal disruption radius by stellar evolution while in an eccentric orbit. After the onset of mass transfer, these stars episodically overflow their Roche lobes every pericenter passage giving rise to low-level flares that repeat on the orbital timescale. Giant stars will exhibit a runaway response to mass loss and "spoon-feed" material to the BH for tens to hundreds of orbital periods. In contrast to full tidal disruption events, the duty cycle of this feeding mode is of order unity for BHs with mass greater than approximately 10 million solar masses. This mode of quasi-steady SMBH feeding is competitive with indirect SMBH feeding through stellar winds, and spoon-fed giant stars may play a significant role in determining the quiescent luminosity of local SMBHs.
1307.3049
A comparative study of radio halo occurrence in SZ and X-ray selected galaxy cluster samples
Sommer, Basu
Aim to make an unbiased census of the radio halo population in galaxy clusters and test whether current low number counts of radio haloes have been a result of selection biases. Construct near-complete samples of galaxy clusters based on public X-ray and SZ cluster catalogs, and search for diffuse, extended (Mpc-scale) emission near their center by analyzing data from the NRAO very large array sky survey. Remove point sources using a matched filtering algorithm, and model the diffuse emission by radial fitting and direct integration. Use and extensive suite of simulations to check for biases. Model the relation between mass and radio halo power with a power law, accounting for the possibility of a population hosting no radio halo emission. Constrain the fraction of targets hosting an extended emission at their centers at 1.4 GHz. Find power law slopes consistent with previous works, though findings suggest the fraction of targets hosting radio haloes may have to be revised upward when considering clusters selected in the SZ effect: ~65% of X-ray selected targets are found to contain no extended radio emission, the corresponding fraction in the SZ selected samples is roughly 20%. The "radio-quiet" population in the X-ray selected cluster samples is in agreement with previous findings. Propose a simple explanation for the observed selection difference based on the distinct time evolution of the SZ and X-ray observables during cluster mergers, and a bias towards relaxed, cool-core clusters in the X-ray selected samples.
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